NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Electric Motor Thermal 

Management R&D

Kevin Bennion

Organization: NREL

Email: kevin.bennion@nrel.gov

Phone: 303-275-4447

Team members/collaborators: 

Justin Cousineau, Charlie King, Gilbert Moreno, Caitlin Stack (NREL)

Tim Burress, Andy Wereszczak (ORNL)

DOE Vehicle Technologies Office

Electric Drive Technologies

FY15 Kickoff Meeting

Oak Ridge National Laboratory

Oak Ridge, Tennessee

November 18 – 20, 2014 This presentation does not contain any 

proprietary or confidential information.

NREL/PR-5400-63004
2

Thermal Management Enables More 

Efficient and Cost-Effective Motors

DOE APEEM FY15 Kickoff Meeting

State of the Art

• Water-ethylene glycol stator 

cooling jacket

• Automatic transmission fluid 

(ATF) impingement on motor 

end windings

Why Motor Cooling

• Current Density

o Size

o Weight

o Cost

• Material Costs

o Magnets

o Rare-earth materials

o Price variability

• Reliability

• Efficiency

Stator Cooling Jacket

Stator End Winding

Rotor

Stator

Photo Credit: Kevin Bennion, NREL

Photo Credit: Kevin Bennion, NREL
3

Passive and Active Cooling

DOE APEEM FY15 Kickoff Meeting

Passive 

Thermal 

Stack

Active 

Convective 

Cooling

Motor

Thermal 

Management

Passive Thermal Stack

•Motor geometry

•Material thermal properties

•Thermal interfaces

Active Convective Cooling

•Cooling location

•Heat transfer coefficients

•Available coolant

•Parasitic power

Photo Credit: Kevin Bennion, NREL
4

Challenges

DOE APEEM FY15 Kickoff Meeting

Stator 

Laminations

Slot Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View

4

1

1

2 3

5

6

Problem

Extracting heat from within the 

motor to protect the motor and 

enable high power density

Challenges

1. Orthotropic (direction 

dependent) thermal conductivity 

of lamination stacks

2. Orthotropic thermal conductivity 

of slot windings

3. Orthotropic thermal conductivity 

of end windings

4. Convective heat transfer 

coefficients for ATF cooling 

5. Thermal contact resistance of 

stator-case contact

6. Cooling jacket performance
5

Proposed Research Objectives

DOE APEEM FY15 Kickoff Meeting

Problem

Core Thermal Capabilities 

and Research Tasks

Objective

Support broad industry demand 

for data, analysis methods, and 

experimental techniques to 

improve and better understand 

motor thermal management

Stator 

Laminations

Slot Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End Winding

Motor Axis

Motor Cooling Section View
6

Research Focus

DOE APEEM FY15 Kickoff Meeting

• Measure convective heat transfer 

coefficients for ATF cooling of end 

windings

• Measure interface thermal resistances 

and orthotropic thermal conductivity 

of materials

Support broad industry demand for data 

to improve and better understand motor 

thermal management

Objective

Research 

Tasks

Automatic Transmission 

Fluid Heat Transfer

Material and Thermal 

Interface Testing

Photo Credit: Jana Jeffers, NREL

Photo Credit: Justin Cousineau, NREL
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Active Cooling

DOE APEEM FY15 Kickoff Meeting

Measure heat 

transfer coefficients 

for ATF cooling of 

end windings

Direct Impingement on 

Target Surfaces

Impingement on Motor End 

Windings

Average Heat Transfer Coefficients

• Establish credibility of experiment and data through comparison of plain target 

surface results to existing correlations in literature

• Produce new data for textured surfaces representative of end-winding wire 

bundles

Spatial Mapping of Convective Heat Transfer Coefficient

• Jet local convective heat transfer 

• Large-scale end-winding convective heat transfer mapping

Photo Credit: Jana Jeffers, NREL Photo Credit: Kevin Bennion, NREL

Accomplishments to Date – FY14
8

ATF Impingement Target Surfaces

DOE APEEM FY15 Kickoff Meeting

Baseline 18 AWG 22 AWG 26 AWG

Radius (wire and insulation), 

mm

N/A 0.547 0.351 0.226

Total wetted surface area, mm2 126.7 148.2 143.3 139.2

• ATF impingement baseline target is plain, polished copper with 600-grit sandpaper

• Additional targets mimic wire bundles with insulation (18, 22, and 26 AWG)

AWG = American Wire Gauge

18 AWG 22 AWG 26 AWG

Credit: Gilbert Moreno, NREL
9

Comparison to Plain Surface Correlations

0

2,000

4,000

6,000

8,000

10,000

12,000

0 2 4 6 8 10

Heat transfer coefficient (W/m

2-K)

Jet velocity (m/s)

50°C

70°C

90°C

Metzger et al., 1974 (90°C)

Ma et al., 1997 (50°C)

Ma et al., 1997 (70°C)

Ma et al., 1997 (90°C)

Leland and Pais, 1999 (50°C)

Leland and Pais, 1999 (70°C)

Leland and Pais, 1999 (90°C)

DOE APEEM FY15 Kickoff Meeting

Comparison of test results to literature correlations on flat target surface

ATF fluid properties provided by Ford
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Spatial Mapping of Heat Transfer

DOE APEEM FY15 Kickoff Meeting

• Jet impingement heat 

transfer coefficients are not 

uniform over the entire 

cooled surface

• The highest heat transfer 

coefficients occur at the jet 

impact zone

• The rate of decrease in the 

heat transfer coefficient is 

unknown

Heated surface

Stagnation zone

Wall jet zone

d, nozzle 

diameter 

Tw

Liquid jet

Air

r / d

Heat transfer coefficient

Local convective heat transfer coefficient
11

TLC Experimental Apparatus

• Fabricated a test section to spatially 

measure jet impingement heat transfer 

coefficients on a discrete heat source.

• Initial tests will be conducted using 

thermochromic liquid crystals (TLCs), 

followed by infrared imaging

DOE APEEM FY15 Kickoff Meeting

vessel

ATF 

inlet

2-mm-diameter 

nozzle

Test article 

fixture

Foil: heated via joule 

heating

Lexan 

insulation/support

Bus bars

Test article cross-sectional view

Teflon insulation/ 

support

Test article bottom view

Bottom side 

view of heated 

foil with camera
12

Passive Thermal Stack

Measure interface 

thermal resistances and 

orthotropic thermal 

conductivity of materials

DOE APEEM FY15 Kickoff Meeting

Material 

Measurements

Effective Thermal Properties for 

Motor Design and Simulation

Stacked lamination thermal conductivity

• Quantified thermal contact resistance between motor laminations

• Supports improved thermal models for motor design and thermal analysis

Winding effective thermal properties

• Initiated work to measure orthotropic thermal properties

• Supports improved thermal models for motor design

• Enables analysis to improve thermal properties of motor materials

Case-to-stator thermal contact resistance

• Developed ability to measure case-to-stator thermal contact resistance

Photo Credit: Justin Cousineau, NREL Photo Credit: Kevin Bennion, NREL

Accomplishments to Date – FY14
13

Through-Stack Thermal Conductivity

DOE APEEM FY15 Kickoff Meeting

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

Cold Plate

Hot Plate

Copper Spreader

Copper Spreader

Copper Metering Block

Copper Metering Block

Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

Photo Credit: Justin Cousineau, NREL
14

Through-Stack Thermal Conductivity

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

0

500

1000

1500

2000

2500

0 2 4 6 8 10

Measured Lamination Stack 

Thermal Resistance (mm2-K/W]

Number of Contacts

M19 29 Gauge, 138 kPa Data

Data with Systematic Error

Weighted Curve Fit

• Lamination-to-lamination thermal contact resistance calculated from 

slope of weighted curve fit

Cold Plate

Hot Plate

Copper Spreader

Copper Spreader

Copper Metering Block

Copper Metering Block

Lamination Stack

Thermal Grease

Thermal Grease

Force Applied

Error bars represent 95% confidence level

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DOE APEEM FY15 Kickoff Meeting
15

Through-Stack Thermal Conductivity

• The lamination-to-lamination thermal contact resistance is affected by the surface 

topography and contact pressure

0

50

100

150

200

250

300

350

138 276 414 552

Lamination-to-Lamination 

Thermal Contact Resistance 

[mm2-K/W]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7

Error bars represent 95% confidence level

15

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

DOE APEEM FY15 Kickoff Meeting
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Through-Stack Thermal Conductivity

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

3.1

3.3

Through

0 10 20 30 40 50

-Stack Thermal Conductivity 

[W/m-K]

Number of Laminations (NL)

M19 29 Gauge, 138 kPa Example

Effective Through-Stack Thermal Conductivity

Asymptote (NNLL → ∞)

• The effective through-stack thermal conductivity approaches the 

asymptote within 30–50 laminations

16

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

DOE APEEM FY15 Kickoff Meeting
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Through-Stack Thermal Conductivity

DOE APEEM FY15 Kickoff Meeting

17

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 200 400 600

Asymptotic Through-Stack Thermal 

Conductivity [W/m-K]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7

Error bars represent 95% confidence level

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

• Asymptotic thermal 

conductivity increases with 

pressure

• Asymptotic thermal 

conductivity depends on 

inter-lamination thermal 

contact resistance, 

lamination thermal 

conductivity, and lamination 

thickness
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In-Plane Lamination Thermal Conductivity

DOE APEEM FY15 Kickoff Meeting

1. Based on measured thermal conductivity of similar material

2. Calculated assuming 99% stacking factor

3. Average of measured orthotropic property in setup shown in figure

21.9 21.7 21.8

0.0

5.0

10.0

15.0

20.0

25.0

Bulk

Material

Calculated

Value

Measured

In-Plane Thermal Conductivity [W/m-K]

M19 29 Gauge Laminations

1 2

3

Photo Credit: Justin Cousineau, NREL

• Confirmed in-plane thermal 

conductivity is close to bulk 

material thermal 

conductivity
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FY15 Tasks to Achieve Key Deliverable

DOE APEEM FY15 Kickoff Meeting

2014

Oct Nov Dec

2015

Jan Feb Mar Apr May Jun Jul Aug Sep

ATF impingement experiments to 

measure variation in local convective 

heat transfer coefficients

Deliverable

Publications 

for EndWinding Heat 

Transfer

Go/ 

No-Go

Select passive 

thermal materials 

for bench-level 

testing in 

partnership with 

ORNL

End-winding spatial mapping of ATF convective heat 

transfer coefficients on end-winding features

Thermal measurements of passive thermal stack 

elements in collaboration with Oak Ridge National 

Laboratory (ORNL)

• Winding thermal properties

• Contact interfaces

• Thermal analysis support in collaboration with ORNL 

motor research
20

Local ATF Jet Heat Transfer Mapping

111°C

64°C

Bottom view of test article Heated foil temperature distribution

Thermal-electrical finite element analysis (FEA) reveals edge effects mostly associated with 

copper bus bars

• ∆T across foil 47°C (most of heat loss to copper bars)

• 85% of foil heater power dissipated by jet 

DOE APEEM FY15 Kickoff Meeting

Spatially measure jet impingement heat transfer coefficients on a discrete heat source.
21

Local ATF Jet Heat Transfer Mapping

Bottom view of test article Heated foil temperature distribution

Use of guard heaters minimize edge effects through the use of guard heaters

• ∆T across foil 7°C (heat loss to copper decreased)

• 98% of foil heater power dissipated by jet

Guard heaters 112°C

105°C

DOE APEEM FY15 Kickoff Meeting

Spatially measure jet impingement heat transfer coefficients on a discrete heat source.
22

Spatial Mapping of Heat Transfer

• Map the large-scale spatial 

distribution of the heat 

transfer coefficients over 

motor end windings

• Study effects of

o Oil jet placement

o ATF free flow over endwinding surfaces

o Jet interactions

Large-scale end-winding convective heat transfer mapping

Photo Credit: Kevin Bennion, NREL

DOE APEEM FY15 Kickoff Meeting
23

Spatial Mapping of Heat Transfer

• Target surfaces installed on 

three surfaces to measure 

heat transfer coefficients

o Outside diameter surface

o Inside diameter surface

o Axial end surface

Custom heat 

transfer sensor 

package in end 

winding

Large-scale end-winding convective heat transfer mapping

Example 

ATF jets

DOE APEEM FY15 Kickoff Meeting
24

Spatial Mapping of Heat Transfer

DOE APEEM FY15 Kickoff Meeting

ATF jets

2

1

Large-scale end-winding convective heat transfer mapping

1. Fluid Jet Geometry

• Location and 

orientation of ATF 

fluid jets

• Nozzle type/geometry

• System flow rate

• Jet velocity

• Parasitic power

2. Relative positon between 

measured heat transfer 

and jet location

• Impact of gravity and 

free fluid flow

• Fluid interactions 

between jets 
25

Passive Thermal Stack

DOE APEEM FY15 Kickoff Meeting

• Case thermal contact resistance

o Case surface characteristics

• Slot and end-winding effective 

thermal conductivity 

• Potting materials for end-winding 

cooling 

Sample Case-Stator Contact 

Surface for Motor Cooling

Example Interference Fit with 

Sample Motor End Windings Stator and Cooling Jacket Case

Photo Credits Kevin Bennion, NREL
26

Publications:

K. Bennion and J. Cousineau, “Sensitivity Analysis of Traction Drive Motor Cooling,” in IEEE Transportation 

Electrification Conference and Expo (ITEC), 2012, pp. 1–6.

Project Summary

DOE APEEM FY15 Kickoff Meeting

Project Duration: FY14–FY16

Overall Objective: Provide data to support broad Industry demand for improving motor thermal 

management

FY14 Focus: Characterized ATF impingement average heat transfer coefficients and completed lamination-tolamination thermal contact resistance measurements

Deliverable: 

Completed measurement of average convective heat transfer coefficients with ATF and compared results of plain 

surface data to correlations in the open literature. Completed lamination-to-lamination thermal contact 

measurements and stack effective thermal conductivity

Go/No-Go: 

Developed test apparatus to measure local impingement heat transfer results with ability to measure local heat 

transfer coefficients. Developed test hardware design and plan for representative wire bundle geometries

FY15 Focus: Spatially map ATF impingement convective heat transfer coefficients and measure passive thermal 

stack thermal resistances

Deliverable: Publish end-winding heat transfer performance data

Go/No-Go: In collaboration with ORNL select passive thermal stack materials for bench-level thermal testing

FY16 Focus: Confirm convective heat transfer performance on in-situ motor thermal performance tests.

Deliverable: Report heat transfer performance and model validation

Go/No-Go: Complete in-situ motor thermal tests for ATF cooling
27

Technology-to-Market Plan

DOE APEEM FY15 Kickoff Meeting

• This research impacts industry needs because ...

o The heat flow, temperature distribution, and fluid dynamics for motor 

thermal management are complex problems

o Data on cooling convective heat transfer coefficients and heat spreading 

within the motor are needed to improve motor performance within cost, 

efficiency, and reliability constraints

• This research is on the path to commercialization because ...

o The research is reviewed by and shared with industry experts

o The data are used by industry experts in the analysis and design of electric 

motors
28

Partners/Collaborators

DOE APEEM FY15 Kickoff Meeting

• Industry

o Motor industry suppliers, end users, and researchers

– Input on research and test plans

– Sharing of experimental data, modeling results, and analysis methods

– Companies providing research input, requesting data, or supplying data 

include: Ford, Chrysler, GM, Tesla, UQM Technologies, Remy, Magna, 

John Deere, Oshkosh

• Other Government Laboratories

o ORNL

– Support from benchmarking activities

– Collaboration on motor designs to reduce or eliminate rare-earth 

materials

– Collaboration on materials with improved thermal properties

 Potting materials for end windings for improved heat transfer

 Slot winding materials
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

For more information, contact:

Principal Investigator

Kevin Bennion

Kevin.Bennion@nrel.gov

Phone: (303)275-4447

APEEM Task Leader:

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov

Phone: (303)275-4062

Acknowledgments:

Susan Rogers and Steven Boyd

U.S. Department of Energy

Team Members:

Justin Cousineau

Charlie King

Gilbert Moreno

Caitlin Stack

Tim Burress (ORNL)

Andy Wereszczak (ORNL)